1. FIELD OF THE INVENTION
The present invention relates to an improvement in a magnetic switch.
2. DESCRIPTION OF THE PRIOR ART
The magnetic switch shown in FIG. 1 has been known as such device. In FIG. 1, the reference numeral (1) designates a magnetic switch: (2) designates a movable iron core having a recess (2a) to which a pinion shift lever (now shown) is fitted; (3) designates a return spring which is fitted to a stepped portion (2b) of the movable iron core (2); and (4) designates a movable contact rod made of a glass filler reinforced nylon formed by molding which is slidably fitted to the fitting hole (6a) of a stationary iron core (6) fixed to a casing (5) and which is slidable in contact with the end of the movable iron core (2) which is moved. The reference numeral (7) designates an electromagnetic coil wound on a bobbin (not shown) which is actuated by closing the key switch of an engine (now shown) to attract the movable iron core (2) in the left direction of the Figure and (8) designates a movable contact which is freely supported on the movable contact rod (4) and is urged by a contact spring (9) in the left direction so as to be in contact with a retaining ring (10) as a stopper. The movable contact rod (4), the movable contact (8), the contact spring (9) and the retaining ring (10) are previously assembled as a movable contact assembly to be fitted into the fitting hole (6a) of the stationary iron core (6). The reference numeral (11) designates a cap assembly which is fixed by crimping to the rear end of the casing (5) through a packing (12) and (13) designates a cap formed by resin molding which holds by bolting a pair of stationary contacts (14) in its two through holes (13a) so as to face the movable contact (8) as shown in FIG. 1. A closed contact circuit is formed by contacting the movable contact. The reference numeral (15) designates a hexagon nut, (16) designates a spring washer, (17) designates a washer and (18) designates a coil spring (mainly used to urge the movable contact assembly) which has one end in contact with the inner surface of the cap as a pressure receiving seat and the other end in contact with the retaining ring (10) to urge it in the right direction. The flange (4a) of the movable contact rod (4) is engaged with the stepped portion (6b) of the stationary iron core (6) by urging force of the coil spring.
The operation of the conventional device will be described. When the electromagnetic coil (7) is actuated by closing the key switch of the engine (now shown), the movable iron core (2) is moved in the left direction against the right side urging force of the return spring to push the movable contact rod (4). In this case, the urging force in the left direction is sufficiently larger than urging force of the coil spring (18) whereby the movable contact (8) is brought into contact with the stationary contact (14) to form a closed contact circuit. When the closed contact circuit is formed, the movable contact rod (4) is moved in the left direction for a small distance such as 1 to 2 mm because of dimensional allowance of the parts of the device. However, this movement is compensated by off-set movement (contact wiping) function by resilient deformation of the contact spring (9). The off-set movement is to self-compensate the closing function of the contacts where the contacts are worn. The electromagnetic coil acts to pull the pinion shift lever (not shown) along with the movable iron core (2) in the left direction. As soon as the key switch is opened, the urging force is released so that the magnetic switch is returned to the original position as shown in FIG. 1 by action of the return spring (3).
In the conventional magnetic switch having the structure described above, it is necessary to form a space for receiving the return spring (3) at the outer portion of the movable contact (2) whereby the diameter of the coiled spring increases. The increased diameter of the coiled spring causes increase of the diameter of the wire of the spring in order to obtain a required urging force thereby increasing a space for the return spring resulting in a large sized device. Also, leakage flux is produced in the stationary iron core (6) to reduce the performance of the magnetic switch.